In recent years, there has been a strong demand for increase in the capacity of magnetic recording apparatus such as a magnetic storage device small in the size and large in the capacity is mounted not only to personal computers, but also to domestic electrical products, where improvements have been required for the recording density. While magnetic heads, magnetic recording media, etc., have been developed vigorously, the mainstream of the recording system has been shifted from the longitudinal magnetic recording system to the perpendicular magnetic recording system and, further, improvement for the recording density has become possible. In the perpendicular magnetic recording medium, it has been known that the efficiency of applying recording magnetic fields from a magnetic head can be improved by adopting a 2-layered perpendicular magnetic recording medium having a soft-magnetic underlayer and this can also cope with the increase of the coersivity of the recording film.
Basically, for providing a role of circulating a recording magnetic field from a magnetic head, it is necessary for the soft-magnetic underlayer to form a thick soft magnetic material of high saturation magnetic flux density (Bs) compared with that of the recording layer and, accordingly, this has resulted in a problem of noises attributable to the soft-magnetic underlayer. The main cause of the noises is attributable to the leakage of magnetic fields from a magnetic domain wall formed in the soft magnetic layer, and a method of suppressing generation of magnetic domain walls or a method of decreasing the leakage of magnetic fields has been studied.
Japanese Patent Publication No. 2001-155321 (“patent document 1”) proposes a method of constituting a soft-magnetic underlayer with two or more soft magnetic layers separated from each other by a non-magnetic layer and using Ru for the nonmagnetic film for separation thereby antiferromagnetically exchange-coupling upper and lower soft magnetic layers. Since the leakage magnetic fields from the magnetic domain wall less occur when the magnetizing direction of the stacked soft magnetic layer is reversed, this provides an effect of suppressing spike noises or demagnetization in recorded magnetization and improving the stray magnetic field resistance with no provision of the magnetic domain control layer. Japanese Patent Publication No. 2004-348777 (“patent document 2”) discloses a method of providing an antiferromagnetic layer or a pinned layer below the exchange-coupled soft-magnetic underlayer thereby controlling the anisotropy of the soft-magnetic underlayer. This is a method of decreasing the leakage magnetic fields by controlling the magnetic domain in the soft-magnetic underlayer and suppressing the magnetic domain wall. However, while the magnetic property of the soft-magnetic underlayer can be controlled, it is not practical to adopt a single magnetic domain structure and the magnetic domain wall cannot be eliminated completely.
On the other hand, Japanese Patent Publication No. Hei 1-267834 (“patent document 3”) proposes provision of a circumferential texture to a substrate for improving the air-bearing property of a magnetic head. Then, as described, for example, in Japanese Patent Publication No. Hei 6-103554 (“patent document 4”), it has been known that when a soft magnetic layer is formed by using a textured substrate, the easy magnetization axis of magnetic anisotropy tends to be directed easily in the direction of the texture by the shape effect.
Further, Japanese Patent Publication No. Sho 63-74121 (“patent document 5”) discloses a method of forming an easy magnetization axis in the direction forming 90° relative to the tensile direction by film-forming the soft magnetic layer having a negative magnetic strain constant while applying tension mechanically to the substrate. Japanese Patent Publication No. Hei 7-235034 (“patent document 6”) shows an example of using a Co series negative magnetic strain soft magnetic material.
A method of exchange-coupling two or more soft magnetic layers is extremely effective in that noises attributable to the soft-magnetic underlayer are decreased. However, it is impossible to adopt the soft magnetic layer into a single magnetic domain structure as described above. In addition, it is difficult to align the direction of the magnetic anisotropy of the soft magnetic layer to one direction by the reasons to be described below. At first, in the current method of forming a film generally by using a magnetron sputtering apparatus, since the leakage magnetic fields from a permanent magnet used as a target cathode of the sputtering apparatus distribute radially, the easy magnetization axis of the soft-magnetic underlayer is provided in the radial direction.
On the other hand, since the substrate is a donut disk apertured at the center, magnetization tends to be directed in the circumferential direction at the inner and the outer circumferences by the shape anisotropy, stress, etc. The effect appears more remarkably in the inner circumference, particularly, as the diameter is smaller. Further, in view of the air-bearing property of the magnetic head, it is preferred to use a substrate textured in the circumferential direction but magnetization further tends to be directed in the circumferential direction by the shape effect. With such composite factors, while the easy magnetization axis is provided relatively in the radial direction from the intermediate circumference to the outer circumference of the substrate, easy magnetization axis is tilted in the circumferential direction at the inner circumference to cause distribution in the anisotropy within the plane of the substrate.
Further, the influence of the shape effect increases in the thin soft-magnetic underlayer. Further, in a soft magnetic layer using exchange-coupling, since the leakage magnetic field in the lower layer reduced with the thickness is small, this results in a problem that the easy magnetization axis of the upper layer is less in antiparallel with the lower layer. Accordingly, when the thickness of the soft-magnetic underlayer formed by using a sputtering apparatus is reduced, the easy magnetization axis tends to be tilted to the circumferential direction from the radial direction.